This invention involves a process for improving the dielectric properties of hermetically-sealed solid-electrolyte tantalum capacitors, and more particularly to improving the storage life of such capacitors which are likely to be stored at elevated temperatures without voltage.
Hermetically sealed solid-electrolyte tantalum capacitors undergo some degree of dielectric degradation when stored at elevated temperatures in the absence of an applied potential. This degradation is manifested after the storage period by a high leakage current upon the re-application of voltage. Typically, a capacitor with a leakage current of less than 0.1 microampere before storage may show leakage current of 1 to 10 microamperes after several days storage at 125.degree. C.
Although high temperature storage for extended periods leads to dielectric damage, this damage can be easily repaired when voltage is subsequently applied, provided that there is sufficient moisture within the capacitor structure to accomplish the necessary electrochemical change.
The role of water as an essential substance in the growth of anodic oxides has been described in the scientific literature (Bernard and Florio, J. Electrocomponents Tech., Vol. II, pp, 137-145, 1984). More specifically, the critical nature of water in the growth of anodic oxide in the presence of a solid electrolyte in tantalum capacitors has long been generally known (Smyth, J. Electrochemical Soc., Vol. 113, p. 19, 1966).
Nonhermetically-sealed capacitors do not show a persistently high leakage current after high temperature storage, because moisture from the environment can be transported either through the casing material or into the casing along the lead wires at a rate sufficient to create a concentration of water which is high enough to carry out the function of dielectric repair rapidly and efficiently upon the re-application of potential.
Hermetically-sealed capacitors, on the other hand, do not allow any ingress of moisture. Furthermore, hermetically-sealed capacitors may be depleted of water at the beginning of their storage life because of the processes that are used in sealing. It is conventional in the art to secure a capacitor body in its impervious can with solder, requiring a temperature of about 200.degree. C., which very effectively expels any residual moisture. Thus, upon sealing the eyelet, the anode is in a dry atmosphere that can no longer provide the water that is required to carry out oxide regrowth if such regrowth should prove necessary.